The Chemistry of Photography
The Chemistry of Photography
Photography is, at its core, applied photochemistry: the study of chemical reactions triggered by light. Every technique from the 1839 daguerreotype to modern inkjet printing rests on the same fundamental idea — that certain materials change in a measurable, useful way when struck by photons.
This guide traces the chemistry of image-making from the earliest experiments through the photographic era and into digital and printing chemistry, and includes practical experiments you can do today.
Part 1: Silver and Light
Why Silver?
Silver sits at a chemical sweet spot. Its halide salts — silver chloride (AgCl), silver bromide (AgBr), and silver iodide (AgI) — are almost completely insoluble in water, which makes them easy to deposit in thin, stable layers. But far more importantly, they are photosensitive: absorbed photons excite electrons from the halide ions and, through a cascade of events, leave behind small clusters of metallic silver atoms. These clusters form the latent image — invisible to the eye, but chemically amplified by development into visible silver metal.
No other element combines this photosensitivity with the right balance of chemical stability, tonal range, and ease of processing. Silver’s photographic role was dominant for 170 years.
Silver Halide Formation
Silver halides form immediately when silver nitrate solution meets a soluble halide:
\[\ce{AgNO3(aq) + NaCl(aq) -> AgCl(s) v + NaNO3(aq)}\]
\[\ce{AgNO3(aq) + KBr(aq) -> AgBr(s) v + KNO3(aq)}\]
\[\ce{AgNO3(aq) + KI(aq) -> AgI(s) v + KNO3(aq)}\]
The precipitate color deepens across the series: AgCl is white, AgBr pale yellow, AgI bright yellow. This correlates with increasing sensitivity to light — AgI absorbs further into the visible spectrum than AgCl.
Part 2: The Silver Processes — Historical Sequence
Photogenic Drawings / Salted Paper Prints (1834)
The very first photographs were made by William Henry Fox Talbot using the simplest possible approach. Paper was soaked in table salt solution (NaCl), allowed to dry, then brushed with silver nitrate solution. Where the two met in the paper fibres, silver chloride formed in situ:
\[\ce{NaCl + AgNO3 -> AgCl(in paper) + NaNO3}\]
Objects were pressed against the paper in sunlight. Where light struck, the AgCl reduced slowly to metallic silver (gray-brown). The shadowed areas remained light. No development was needed — the image “printed out” directly.
The problem was fixing. Without removing the unexposed AgCl, continued light exposure turned the whole image dark. Talbot initially used strong salt solution (which forms slightly soluble AgCl complexes). John Herschel showed him that sodium thiosulfate worked far better — a discovery that has remained central to photography ever since.
Herschel’s fixing reaction:
\[\ce{AgCl(s) + 2Na2S2O3(aq) -> Na3[Ag(S2O3)2](aq) + NaCl(aq)}\]
The silver thiosulfate complex is soluble, washing away with water and leaving only the silver image.
The Daguerreotype (1839)
Louis Daguerre announced the first practical photographic process in Paris in January 1839, creating an immediate worldwide sensation. His images had extraordinary sharpness and tonal delicacy — a quality that wet-process photographs still struggle to match.
The chemistry:
A copper plate was polished to a mirror finish and coated with silver (by electrodeposition or by rolling). The silver surface was exposed to iodine vapor, forming a thin layer of silver iodide:
\[\ce{2Ag + I2 -> 2AgI}\]
Later, bromine vapor was added to increase sensitivity. The plate was then loaded into a camera and exposed (initially for many minutes; improved plates brought this down to seconds).
Development used mercury vapour heated to ~60°C. Mercury atoms preferentially bonded to exposed silver — the latent image — forming a bright silver-mercury amalgam that visually amplified the image from invisible to strong:
\[\ce{Ag_{latent} + Hg(v) -> Ag\text{-}Hg_{amalgam}}\]
The image was fixed by washing with sodium thiosulfate solution (or originally, hot salt solution), then rinsed with distilled water and dried.
The result: a unique, unrepeatable positive image of extraordinary sharpness, with no negative. Each daguerreotype is a one-of-a-kind object — you cannot make copies.
Why it died: The mercury development process was dangerously toxic. The images had to be viewed at precisely the right angle (otherwise they appeared as negatives). And critically, because there was no negative, you could not make prints. Each portrait sitting produced exactly one image.
The Calotype / Paper Negative (1841)
Talbot’s improved process used a chemical developer to amplify a latent image — the key invention that underlies all subsequent silver photography. Instead of waiting for light to print out a full image, a brief exposure created only invisible chemical changes; the developer then selectively reduced the exposed silver halide to metallic silver.
Sensitization: Paper was coated with silver iodide (using silver nitrate then potassium iodide), then brushed with “gallo-nitrate of silver” (silver nitrate + gallic acid) immediately before exposure.
Development: After exposure, the paper was warmed and brushed with more gallo-nitrate solution. Gallic acid reduced the exposed AgI:
\[\ce{2AgI + C6H8O7 (gallic acid) -> 2Ag(s) + products}\]
This gave a paper negative (dark where light struck, light where it didn’t). Positive prints were made by pressing the negative against fresh sensitized paper in sunlight — a contact print. This negative-to-positive system became the basis of all film photography.
Fixed with sodium thiosulfate (“hypo”). Washed and dried.
The limitation: The paper grain was visible in the image, limiting fine detail. This was considered an artistic advantage by some (a softness that the daguerreotype lacked) but a practical disadvantage.
The Albumen Print (1850–1895)
The most widely made photograph of the 19th century. The process coated paper with a layer of beaten egg white (albumen) containing dissolved sodium chloride, then sensitized it with silver nitrate. The albumen, when dried, formed a smooth, protein-based layer that held the silver chloride uniformly and gave the characteristic glossy surface of Victorian photographs.
\[\ce{NaCl_{(in albumen)} + AgNO3 -> AgCl_{(in albumen)} + NaNO3}\]
Albumen prints were contact-printed (negative pressed against paper) under sunlight until the image “printed out” to the desired depth — no development needed. The silver chloride slowly reduced directly under light exposure. Fixed with sodium thiosulfate, toned with gold chloride (which improved archival stability and gave the characteristic warm purple-brown color by replacing silver with gold).
Gold toning:
\[\ce{3Ag + AuCl3 -> Au + 3AgCl}\]
More stable gold metal replaced less stable silver, while the released silver chloride was washed away or fixed. The characteristic warm tone of victorian portraits comes from this gold.
Billions of albumen prints were made between 1855 and 1895. The demand for albumen created an entire industry — Dresden alone consumed six million eggs per year for photographic paper production in the 1860s.
Wet Plate Collodion (1851)
Frederick Scott Archer’s process combined the sharpness of the daguerreotype with the reproducibility of the calotype, and became the dominant photographic medium for thirty years.
Collodion is gun cotton (nitrocellulose) dissolved in ether and alcohol — a syrupy, fast-drying liquid that forms a tough, transparent film. The photographer poured it onto a clean glass plate, tilted to coat evenly, then immersed the still-tacky plate in silver nitrate solution:
\[\ce{2KI_{(in collodion)} + 2AgNO3 -> 2AgI_{(in collodion)} + 2KNO3}\]
The plate was loaded immediately into the camera while still wet (the collodion must not dry — dried collodion becomes impermeable to the developing chemistry). Exposed, then developed within ten minutes using pyrogallic acid or ferrous sulfate solution, which reduced the exposed silver iodide to metallic silver:
\[\ce{2AgI + FeSO4 -> 2Ag(s) + FeI2 + SO4^{2-}}\]
Fixed with potassium cyanide (toxic) or sodium thiosulfate, washed, varnished to protect.
This process required the photographer to carry a complete darkroom — glass plates, collodion, silver nitrate bath, developing chemicals, washing water. Field photographers pulled a horse-drawn darkroom wagon to battle sites and landscapes. Despite this inconvenience, the images were exquisitely sharp and could be produced in any quantity.
The ambrotype was a wet plate collodion negative, slightly underexposed and mounted on black velvet or paper. Against the dark background, the negative tones appeared reversed — producing a positive image. A cheap, quick alternative to the daguerreotype.
The tintype (ferrotype) used the same chemistry on a black-japanned iron sheet instead of glass — even cheaper, durable, and could be cut to any size. Tintypes were made by street photographers into the 1930s.
Dry Plate (Gelatin Silver) (1871)
Richard Leach Maddox’s discovery that silver bromide could be suspended in gelatin — and that the dried, gelatin-coated plate retained its photosensitivity — transformed photography from a craft into an industry.
Gelatin silver bromide emulsions could be coated on glass, dried, stored for months, shipped anywhere, and exposed at any time. The wet darkroom wagon was no longer needed. Exposure times fell to fractions of a second as emulsion chemistry was refined. The Eastman Kodak company adapted the gelatin emulsion to flexible celluloid film in 1889, making the hand camera and snapshot photography possible.
The chemistry of gelatin emulsions remained the basis of photography — on glass, cut film, roll film, sheet film, and motion picture film — until digital imaging replaced silver halide in the early 2000s for most applications.
Part 3: The Chemistry of Silver Photography in Detail
The Latent Image
When a photon is absorbed by a silver halide grain, it promotes an electron from the halide ion into the conduction band of the crystal. This mobile electron migrates through the crystal lattice until it is trapped at a sensitivity speck — a minute imperfection or silver sulfide cluster introduced deliberately during emulsion manufacture.
The trapped electron reduces a silver ion to a silver atom:
\[\ce{Ag+ + e- -> Ag^0}\]
One silver atom is unstable. Two are slightly more stable. A cluster of three or four atoms becomes stable enough to persist as the latent image. The process repeats at many sensitivity specks across the grain with each additional photon. A grain with four or more silver atoms becomes developable; one with fewer does not.
This is why photography works: a latent image of just a few dozen silver atoms per grain can be chemically amplified into billions during development — a sensitivity gain of around 10⁸.
Development
Developers are reducing agents that donate electrons to silver ions, reducing them to metallic silver. The key is selectivity: the developer must preferentially reduce exposed grains (those with a latent image) while leaving unexposed grains largely intact.
Silver ions are reduced much more rapidly at an existing metallic silver catalyst. The latent image acts as a catalyst for the reduction of the entire grain.
Common developers:
| Developer | Type | Character |
|---|---|---|
| Metol (monomethyl-p-aminophenol) | Amine | Soft tones, fine grain |
| Hydroquinone (1,4-benzenediol) | Phenol | High contrast |
| Pyrogallol | Tannin | Staining developer, beautiful tones |
| Ascorbic acid (vitamin C) | Reducing acid | Modern, non-toxic alternative |
| Ferrous sulfate | Iron salt | Wet plate collodion, simple |
Generic development reaction (metol as example):
\[\ce{2Ag+ + Metol_{red} -> 2Ag^0 + Metol_{ox}}\]
Stop Bath
Development is halted by a dilute acid rinse (typically acetic acid). The acid neutralises the alkaline developer still present on the film/paper surface:
\[\ce{CH3COOH + NaOH -> CH3COONa + H2O}\]
Without a stop bath, development continues as the film is moved to the fixer, causing overdevelopment in some areas.
Fixer
The fixing solution dissolves unexposed silver halide, making the image permanent and stable in light.
Sodium thiosulfate (Na₂S₂O₃, “hypo”) has been the standard fixer since Herschel’s recommendation in 1839:
\[\ce{AgBr(s) + 2Na2S2O3(aq) -> Na3[Ag(S2O3)2](aq) + NaBr(aq)}\]
The silver thiosulfate complex dissolves readily in the fixer solution and washes out completely in the subsequent water wash.
Ammonium thiosulfate works faster and is used in “rapid fixers.” Both are identical in principle.
Over-fixed film becomes weak and eventually starts to bleach the silver image itself, as the fixer attacks the developed silver too. Under-fixed film retains milky silver halide in the shadows, which eventually darkens on exposure to light.
Washing and Archival Processing
After fixing, the film or paper must be washed thoroughly to remove thiosulfate, which would slowly oxidize and stain the image. The archival permanence of a silver photograph depends almost entirely on how well it was washed and whether it was toned.
Hypo clearing agent (sodium sulfite solution) converts residual thiosulfate into more water-soluble compounds, reducing the wash time needed for archival permanence.
Selenium toning converts silver metal to silver selenide, which is more stable and darker. It also gives a cooler color to prints. Widely used for archival processing.
Summary: The Complete Silver Photographic Cycle
| Step | Chemistry | Purpose |
|---|---|---|
| Sensitization | AgNO₃ + halide → AgX (silver halide in emulsion) | Creates light-sensitive layer |
| Exposure | AgX + hν → Ag⁰ (latent image, 4–50 atoms) | Records the image |
| Development | Ag⁺ + developer → Ag (full grain, ×10⁸ amplification) | Makes image visible |
| Stop bath | Developer + acid → neutralised developer | Halts development |
| Fixing | AgX + thiosulfate → soluble Ag complex | Removes unexposed silver, stabilises image |
| Washing | Water → removes thiosulfate and complexes | Ensures archival permanence |
| Toning (optional) | Ag + toner → AgSe, AuAg, or Ag₂S | Increases stability, alters colour |
Part 4: Alternative (Non-Silver) Processes
Cyanotype (1842)
The cyanotype is the simplest and most accessible historical photographic process. It was invented by Sir John Herschel in 1842, the same man who gave photography the words “positive,” “negative,” and “snapshot,” and who showed Talbot how to fix photographs with thiosulfate.
The sensitizer is a mixture of two iron(III) compounds: ferric ammonium citrate and potassium ferricyanide. UV light reduces Fe³⁺ to Fe²⁺ in the exposed areas:
\[\ce{Fe^{3+} + e^{-} ->[\text{UV}] Fe^{2+}}\]
The Fe²⁺ then reacts with the ferricyanide to form insoluble Prussian blue (iron(III) hexacyanoferrate(II) — the same pigment synthesised in the Prussian Blue experiment):
\[\ce{Fe^{2+}_{(aq)} + [Fe(CN)_6]^{3-}_{(aq)} -> KFe[Fe(CN)_6]_{(s)}}\]
Unexposed areas retain the soluble iron salts, which wash away cleanly with water, leaving the paper white. The result is a bold cyan-blue image on white paper.
The botanist Anna Atkins used cyanotype in 1843 to produce Photographs of British Algae: Cyanotype Impressions — the first book illustrated with photographs. She pressed algae specimens directly against coated paper, exposed them to sunlight, and washed the prints in water. Her contact prints (photograms) are among the most beautiful images in photographic history.
The same process was used for architectural blueprints from the 1870s until the 1950s: engineers’ drawings were traced onto translucent paper, placed in contact with cyanotype-coated paper, and printed in sunlight. The word “blueprint” comes from this process.
Cyanotype Sensitizer Recipe
| Component | Amount | Function |
|---|---|---|
| Ferric ammonium citrate (green) | 20g | Dissolved in 100mL water (Solution A) |
| Potassium ferricyanide | 8g | Dissolved in 100mL water (Solution B) |
Mix equal parts of A and B immediately before use. The mixed solution keeps for a few hours; store A and B separately (stable for months in dark bottles).
Cyanotype Procedure
- In dim light, brush or sponge a thin, even coat of mixed sensitizer onto watercolour paper or fabric.
- Allow to dry in darkness (20–40 minutes, or use a hair dryer on cool setting).
- Arrange objects or a transparency negative on the coated surface. Press firmly under glass if possible.
- Expose to direct sunlight or UV lamp. Sunlight: 5–20 minutes depending on brightness. UV lamp (365nm): 3–10 minutes. The coated surface should shift from yellow-green to grey-blue/silver during exposure.
- Rinse thoroughly in running water for 2–5 minutes. The blue develops rapidly as the image oxidizes in water. Rinse until the wash water runs clear.
- Allow to dry. The image deepens slightly as it dries and oxidizes fully.
What works well: Flat botanical specimens (leaves, flowers, feathers), lace, keys, circuit boards, hands, negatives on transparent film.
Cyanotype Chemistry Notes
- Hydrogen peroxide (a few drops in the wash water) accelerates the final oxidation, giving a deeper, faster blue.
- Ammonia or baking soda can partially bleach cyanotypes (turns them yellow-brown). This is reversible: re-exposing the bleached print to air restores the blue over hours or days.
- Cyanotypes are lightfast (very stable to fading) but sensitive to alkalis. Do not frame behind alkaline mat board.
- UV sensitivity means overcast days work fine but take longer.
Van Dyke Brown Print (1895)
The Van Dyke process uses the same photoreduction of Fe³⁺ to Fe²⁺, but the Fe²⁺ then reduces silver ions (from silver nitrate in the sensitizer) to metallic silver, giving warm brown tones instead of blue.
\[\ce{Fe^{3+} ->[\text{UV}] Fe^{2+}}\]
\[\ce{Fe^{2+} + Ag^{+} -> Fe^{3+} + Ag^0 (\text{brown/sepia})}\]
Sensitizer:
| Component | Amount |
|---|---|
| Ferric ammonium citrate | 9g in 33mL water |
| Tartaric acid | 1.5g in 33mL water |
| Silver nitrate | 3.8g in 33mL water |
Mix in order just before use.
After UV exposure, wash briefly in water, fix in sodium thiosulfate (10% solution) for 1 minute, then wash again thoroughly. The fixer removes unexposed silver nitrate, leaving the brown metallic silver image.
Van Dyke prints have a beautiful warm brown-to-sepia tone that suits portraits and botanical studies. Less archival than cyanotype (silver is susceptible to tarnish) but historically important and visually distinctive.
Platinum and Palladium Printing (1873)
Platinum and palladium prints are considered the finest of all photographic processes — prized for their extraordinary tonal range, matte surface, and archival permanence. Platinum and palladium metals do not oxidise; platinum prints made in the 1890s are as fresh today as when they were made.
The photochemistry again uses iron: Fe³⁺ is reduced to Fe²⁺ by UV light. The Fe²⁺ then reduces platinum or palladium ions to their metallic form:
\[\ce{Fe^{3+} ->[\text{UV}] Fe^{2+}}\]
\[\ce{Fe^{2+} + PtCl_6^{2-} -> Pt^0 + Fe^{3+} + 6Cl^-}\]
The image is pure metallic platinum or palladium embedded in the paper fibre — not on the surface, so there is no surface sheen at all. Development is with potassium oxalate solution, which removes the iron salts and completes the reduction.
The cost of platinum and palladium makes this the most expensive alternative process, but photographers who use it regard the results as incomparable.
Gum Bichromate (1894)
Gum bichromate uses the hardening of dichromate-sensitised gum arabic under UV light — not a silver process at all.
Mechanism: Potassium or ammonium dichromate in solution sensitises gum arabic. UV light causes the dichromate ion (Cr₂O₇²⁻) to oxidize the gum, cross-linking the polysaccharide chains and rendering them insoluble in water. Unexposed gum remains soluble and washes away.
\[\ce{Cr_2O_7^{2-} + 3 H_2O + 8 H^+ + 6e^- -> 2Cr^{3+} + 7H_2O}\]
By mixing the gum with any pigment and coating paper, any color image is possible. Multiple printings with different pigments build up a full-color image. The process has a painterly, soft quality loved by pictorialist photographers.
Safety note: Hexavalent chromium compounds (dichromates) are toxic and carcinogenic. Handle with care and dispose appropriately.
Anthotype (Organic, Impermanent)
Some plant pigments are bleached by UV light — chlorophyll, most berry dyes, turmeric. Paper or fabric soaked in these pigments and exposed under a negative will bleach in the light areas while the shaded areas retain their color.
The resulting image is the inverse of the negative: light where the negative was dark, dark where it was transparent.
Anthotypes are not permanent — the same light that made them will continue to bleach them. They can only be viewed briefly in dim light, or preserved by scanning immediately after development. The process is more art project than archival photograph, but it demonstrates photochemistry with nothing but plant extracts and sunlight.
| Pigment | Color | Notes |
|---|---|---|
| Turmeric | Yellow | Fast bleaching, very sensitive |
| Beet juice | Red-pink | Medium speed |
| Spinach (chlorophyll) | Green | Slow, requires long exposure |
| Red cabbage | Purple | pH sensitive as well as light-sensitive |
| Rose petals | Pink | Classic choice |
Part 5: Printing and Reproduction Chemistry
Photogravure (1879)
Photogravure uses the same dichromate-gelatin hardening principle as gum bichromate, but to etch a copper printing plate rather than make a direct print.
Carbon tissue (gelatin + potassium dichromate) is exposed under a UV-transparent positive transparency. Exposed areas harden; unexposed areas remain soft. The tissue is pressed onto a copper plate, and the soft gelatin is washed away, leaving a relief of hardened gelatin of varying thickness. The plate is then etched with ferric chloride, which attacks unprotected copper in proportion to how thin the remaining gelatin is.
The etched plate is wiped with ink and printed on damp paper under a press. The varying depth of the etched wells holds varying amounts of ink, reproducing a continuous tone image with extraordinary quality.
Photogravure was the standard method for reproducing photographs in books and magazines from the 1880s until the 1960s. The quality is regarded as superior to any other method of ink-on-paper reproduction.
Lithography and Photolithography
Traditional lithography (1796, Alois Senefelder) exploits the immiscibility of grease and water. A greasy drawing on a flat stone or metal plate is dampened with water, then inked: ink (oil-based) adheres to the greasy image areas but is repelled by the wet blank areas. Pressed onto paper, it transfers the image.
Photolithography added a UV-sensitive coating to the plate: a solution of asphalt or gum arabic sensitised with dichromate. UV exposure through a photographic transparency hardens or softens the coating, which is then developed to leave the plate ready for inking.
Modern offset lithography — the basis of all commercial printing — uses photopolymer plates where UV-sensitive resins harden or soften on exposure. The same principle applies: exposure defines what will accept ink.
Semiconductor photolithography (used to make computer chips) is a direct descendant of this chemistry, using photoresists — polymers that change solubility on UV or X-ray exposure — to define the nanometre-scale circuit features on silicon wafers. Every chip in every phone, computer, and camera was made with photographic chemistry.
Screen Printing with Photoemulsion
Silk screen or screen printing uses a mesh stretched over a frame. Normally, ink is pushed through the entire mesh by a squeegee. To make a screen print of a specific image, the mesh is coated with photosensitive emulsion, exposed through a positive transparency, and developed to block the unexposed areas.
Photoemulsion is typically polyvinyl alcohol sensitised with ammonium dichromate or a diazo compound. UV exposure cross-links the polymer; unexposed emulsion washes away with water, leaving open mesh in the image areas and blocked mesh in the background.
The same dichromate chemistry as gum bichromate and photogravure — but used to make a reusable printing screen.
Part 6: Darkroom Chemistry — Practical Reference
Film Development
Standard black and white film processing follows a precise sequence:
| Step | Chemical | Time | Purpose |
|---|---|---|---|
| Developer | See table below | Per film/temperature | Reduces exposed silver |
| Stop bath | 2% acetic acid | 30–60 seconds | Neutralises developer |
| Fixer | Sodium or ammonium thiosulfate | 3–10 minutes | Removes unexposed silver halide |
| Hypo clear (optional) | Sodium sulfite | 1–2 minutes | Aids washing |
| Wash | Running water | 20–30 minutes | Removes fixer |
| Photo-flo (optional) | Wetting agent | 30 seconds | Prevents drying marks |
Common Black and White Developers
| Developer | Type | Grain | Contrast | Notes |
|---|---|---|---|---|
| D-76 (Kodak) | Metol/hydroquinone | Fine | Normal | Industry standard |
| HC-110 | Metol | Very fine | Normal | Concentrated, economical |
| Rodinal (Adonal) | para-aminophenol | Larger | High | Classic, makes sharper grain |
| Pyrocat-HD | Pyrocatechin | Very fine | Normal | Staining developer, archival |
| Caffenol | Coffee + ascorbic acid + washing soda | Medium | Variable | Home brew developer — see below |
Caffenol: The Coffee Developer
Caffenol demonstrates that development is just reduction chemistry — and that reducing agents are everywhere.
Coffee contains caffeic acid and other polyphenol reducing agents. Ascorbic acid (vitamin C) is a strong reducing agent. Sodium carbonate (washing soda) creates the alkaline environment required for development.
Caffenol-C recipe (per 500mL):
| Component | Amount | Function |
|---|---|---|
| Washing soda (Na₂CO₃, anhydrous) | 16g | Alkaline activator |
| Ascorbic acid | 6g (dissolve first) | Primary reducing agent |
| Instant coffee | 40g (dissolve last) | Secondary reducing agent + anti-foggant |
| Water | 500mL (room temperature) |
Development times: 15–20 minutes at 20°C for most films, with gentle agitation every minute. Fix normally with sodium thiosulfate.
The coffee phenols develop the image with coarser, more acutate grain than conventional developers. The ascorbic acid prevents chemical fog (background graying from unexposed grains being reduced). The result is a perfectly usable negative with a slightly gritty character.
Part 7: Home Experiments
Experiment: Cyanotype Photogram
Make photographic images without a camera, using sunlight and chemistry.
Difficulty: Easy | Time: 30 minutes | Visual Impact: Very High
Materials: - Ferric ammonium citrate (green) — 20g in 100mL water (Solution A) - Potassium ferricyanide — 8g in 100mL water (Solution B) (or pre-mixed cyanotype sensitizer) - Watercolour paper (90lb or heavier) or cotton fabric - Flat objects to print: leaves, flowers, feathers, keys, lace, circuit boards - A sunny day, or UV lamp - Glass sheet (optional, to press objects flat) - Tray of water for development
Procedure: 1. Mix equal volumes of Solution A and Solution B just before use. Work under normal artificial light, but avoid bright sunlight or UV. 2. Brush a thin, even coat onto the paper using a foam brush or sponge. Coat in one direction, then cross-coat perpendicular for even coverage. 3. Hang to dry in a dark room or in shade. The paper should turn pale yellow-green. 4. Arrange objects face-down on the dried paper. Cover with glass if possible for sharp edges. 5. Carry the whole assembly to bright sunlight. Expose 5–20 minutes (strong summer sun: 5 min; hazy or winter: 15–20 min). Watch the exposed areas shift from yellow-green to grey-blue. 6. Take indoors and quickly rinse under running cold water for 2–3 minutes. The image appears as the deep cyan-blue Prussian blue forms in the light areas. 7. Let dry flat. The image deepens and clarifies as it dries and oxidises in air.
Extensions: - Add a few drops of hydrogen peroxide to the final rinse water to intensify the blue and speed oxidation. - Try the same process on fabric (cotton, linen, or silk) — wash in cold water and allow to air dry. - Make “cyanotype sun prints” by arranging objects and leaving them in place as the sun moves, creating shadows within shadows.
Experiment: Salted Paper Print
The simplest of all silver processes — no darkroom, no enlarger.
Difficulty: Medium | Time: 1 hour (plus printing time in sun) | Visual Impact: High
Safety: Silver nitrate stains skin and surfaces permanently black. Wear gloves throughout.
Materials: - Silver nitrate — 12g in 100mL distilled water (store in dark bottle) - Table salt (NaCl) — 20g in 1 litre distilled water - Watercolour paper - Sodium thiosulfate — 100g in 1 litre distilled water (fixer) - Objects to print or a transparent negative printed on inkjet film
Procedure: 1. Brush or soak paper in salt solution. Allow to dry in normal light (salt solution is not light-sensitive yet). 2. In dim light, brush the dried paper with silver nitrate solution. The silver nitrate and sodium chloride in the paper react to form silver chloride in situ. The paper turns slightly cream-coloured. 3. While still damp, immediately place your object or negative against the paper and press under glass. 4. Expose in direct sunlight. The image will slowly print out — exposed areas turning from pale cream to brown, then gray-brown as metallic silver forms. This takes 10–40 minutes. The image should be somewhat overexposed — it will lighten slightly on fixing. 5. Rinse briefly in water. Fix in sodium thiosulfate solution for 5 minutes (this dissolves unexposed silver chloride, making the image permanent). 6. Wash thoroughly in running water for 30 minutes. Dry flat.
The Science: \[\ce{AgCl ->[\text{UV}] Ag^0 (\text{brown})}\] The silver chloride decomposes photolytically. The metallic silver image is “printed out” by the light itself, without a separate developing step.
Experiment: Caffenol Film Development
Develop black and white photographic film using only coffee, vitamin C, and washing soda.
Difficulty: Medium | Time: 45 minutes | Visual Impact: High
Materials: - Exposed black and white film (any format — 35mm easiest) - Developing tank and reels (Patterson or similar) - Instant coffee (not decaffeinated) — 40g - Ascorbic acid (vitamin C) — 6g - Sodium carbonate (washing soda, Na₂CO₃ anhydrous) — 16g - Sodium thiosulfate — 100g in 500mL water (fixer) - Thermometer
Procedure: 1. In complete darkness (or a changing bag), load the film onto the developing reel and seal in the tank. 2. Dissolve ascorbic acid in 250mL room-temperature water. Dissolve washing soda separately in 100mL warm water, let cool. Dissolve coffee in 150mL water. Combine in this order: water + ascorbic acid → washing soda → coffee. The solution will fizz slightly. Total volume 500mL. 3. Note the temperature — aim for 20°C. At 20°C, develop for 15 minutes. 4. Pour developer into tank. Invert 4 times in first 10 seconds, then 4 inversions every 60 seconds. 5. Pour out developer. Pour in stop bath (or plain water) for 30 seconds. 6. Fix for 5–10 minutes (sodium thiosulfate), agitating occasionally. 7. Wash for 20 minutes. A few drops of washing-up liquid in a final 30-second rinse prevents drying marks. 8. Hang to dry in a dust-free area.
What to expect: The negatives will have good tonality with a slightly gritty, characterful grain. The coffee creates a slight brownish stain in the film base that adds to the look.
Part 8: Summary of Photographic Processes
Timeline of Key Processes
| Year | Process | Inventor | Chemistry | Image Colour |
|---|---|---|---|---|
| 1834 | Photogenic drawing | Fox Talbot | Silver chloride print-out | Warm brown |
| 1839 | Daguerreotype | Daguerre | AgI + mercury development | Mirror silver |
| 1841 | Calotype | Fox Talbot | Silver iodide, gallic acid development | Brown-black |
| 1842 | Cyanotype | Herschel | Fe³⁺/Fe²⁺ → Prussian blue | Cyan blue |
| 1850 | Albumen print | Blanquart-Évrard | Silver chloride in egg white, gold toned | Purple-brown |
| 1851 | Wet plate collodion | Archer | AgI in collodion, ferrous sulfate development | Grey-black |
| 1873 | Platinum print | Willis | Fe³⁺/Fe²⁺ → Pt⁰ or Pd⁰ | Warm-cool grey |
| 1871 | Dry plate | Maddox | AgBr in gelatin | Grey-black |
| 1889 | Roll film | Eastman | AgBr in gelatin on celluloid | Grey-black |
| 1894 | Gum bichromate | Rouillé-Ladevèze | Dichromate-hardened gum + pigment | Any colour |
| 1895 | Van Dyke brown | Arndt & Troost | Fe³⁺/Fe²⁺ → Ag⁰ | Warm brown |
Key Chemicals
| Chemical | Role in Photography |
|---|---|
| Silver nitrate | Source of Ag⁺ for all silver processes; sensitizer for salted paper, albumen, wet plate |
| Sodium thiosulfate | Universal fixer; dissolves unexposed silver halide |
| Ferric ammonium citrate | UV-sensitive iron sensitizer for cyanotype, Van Dyke, platinum |
| Potassium ferricyanide | Ferricyanide component of cyanotype; forms Prussian blue with Fe²⁺ |
| Ferrous sulfate | Wet plate developer; reduces AgI to Ag⁰; component of some traditional developers |
| Ascorbic acid | Modern, non-toxic developer; used in Caffenol and commercial developers |
| Sodium carbonate | Alkaline activator for film developers including Caffenol |
| Potassium dichromate | Sensitizer for gum bichromate, photogravure, photolithography |
| Gallic acid | Fox Talbot’s original calotype developer |
| Gum arabic | Binder in gum bichromate process |
Resources
Books: - The Darkroom Handbook by Michael Langford — comprehensive practical reference - Primitive Photography by Alan Greene — historical processes hands-on guide - The Book of Alternative Photographic Processes by Christopher James — encyclopaedic - Photographic Chemistry by George Glafkides — the technical chemistry in depth - Photography and the Art of Seeing by Freeman Patterson
Websites: - Cyanotype.ca — detailed cyanotype instructions and history - Bostick & Sullivan — alternative process supplies and guides - Caffenol.org — Caffenol developer recipes and film results - APUG / Photrio — community forum for traditional and alternative photography - George Eastman Museum — history of photography, process descriptions